Proton-Coupled Electron Transfer Deoxygenation of Pyridine N-Oxide: A Mechanistic Study.

Electrochemical reductive deoxygenation of pyridine N-oxide is investigated with particular focus on the role of proton-coupled electron transfers. A detailed analysis of cyclic voltammograms reveals that the initial electron transfer is followed by protonation of the pyridine N-oxide anion radical. Kinetic analysis reveals an unusual fifth-order dependence on the concentration of the proton donor (either water or ethanol), suggesting the involvement of a proton donor cluster in the protonation step. The resulting neutral radical represents a key bottleneck in the reaction pathway, as it can proceed via either a parent-child coupling reaction or NO bond cleavage, the latter leading to the formation of pyridine. This competition between reaction pathways allows extraction of both the rate constant for the protonation of the N-oxide radical anion and kinetic information related to the reductive NO bond cleavage. The reductive cleavage of the protonated N-oxide radical may proceed via two possible mechanisms: 1) homolytic bond cleavage followed by reduction of the hydroxyl radical, or 2) a concerted dissociative electron transfer. The observed hydrogen-bonding effects, combined with the higher driving force for the concerted pathway, support the latter mechanism, where stabilization of the departing hydroxide ion facilitates the electron transfer.